Notice that the 1.4x III has less barrel distortion than the 1.4x II. This makes the center-of-the-frame details slightly smaller in a comparison. This difference will be most noticeable in the top crop shown in the ISO 12233 chart tool.

Noticeable in the bottom crop is the reduced CA (Chromatic Aberration) in the 1.4x Extender III.

The Extender 2x III is slightly sharper than the Extender 2x II, but the Extender 1.4x III is not significantly different than the Extender 1.4x II in this regard.